Carbon dioxide, methane, and halocarbons, the primary greenhouse
gases, were discussed in The Carbon Dioxide
Problem so
they will not be discussed further here. The other greenhouse gases,
ozone and nitrous oxides, have other important effects described on this
page.

The Environmental Protection Agency's Six Principal Pollutants.

The Environmental Protection Agency has set national air
quality standards for six principal air pollutants (also called the criteria
pollutants): nitrogen dioxide (NO2), ozone (O3), sulfur dioxide
(SO2), particulate matter (PM), carbon monoxide (CO), and lead (Pb).

Primary standards set
limits to protect public health, including the health of "sensitive" populations
such as asthmatics, children, and the elderly. Secondary
standards set
limits to protect public welfare, including protection against decreased
visibility, damage to animals, crops, vegetation, and buildings. From
Environment Protection
Agency.

Nitrogen
dioxide

Nitrogen dioxide is a reddish-brown, highly reactive gas that is formed
in the ambient air through the oxidation of nitric oxide (NO). It is
produced by motor vehicles (55%), power plants (22%), and other industrial,
commercial, agricultural, and residential sources that burn fuels or
biomass (22%). The higher the combustion temperature, the more nitrogen
is oxidized. Lightning and microbial activity are natural sources of
nitrogen oxides. Nitrogen oxides in the atmosphere react with water,
oxygen, and oxidants to form acidic compounds. These compounds fall to
the Earth in either dry form (gas and particles) or wet form (rain, snow,
and fog) as acid rain.

Oxides of nitrogen can be carried great distances by winds, and
they:

Are one of the main ingredients involved in the formation of
ground-level ozone, which can trigger serious respiratory problems.

React to form nitrate particles, acid aerosols (microscopic particles
in the air), as well as NO2, which also cause respiratory problems.

Contribute to formation
of acid rain.

Contribute to nutrient overload that deteriorates water
quality.

Contribute to atmospheric particles, that cause visibility
impairment most noticeable in national parks.

React to form toxic
chemicals.

Contribute to global warming.

Absorbs sunlight giving air a reddish brown color.

Average density of nitrogen dioxide in the troposphere in 2004. The
image makes clear just how human activities impact air
quality. Click on image for a zoom. Data are from the Scanning Imaging
Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)
instrument on Envisat. From European Space Agency GMES Service Nitrogen
dioxide page.

The Environmental Protection
Agency publishes maps of their Air
Quality Index based on the concentration of these major air pollutants.
There was no area of the country in noncompliance with EPA standards
for nitrogen dioxide in March 2006.

Ozone (O3)

Ozone is Good
Up High Bad Nearby. It is good when it occurs in
the stratosphere, where it absorbs ultraviolet radiation (energy) from
the sun. It is bad when it occurs close to the ground in the troposphere,
where it is a pollutant. Tropospheric ozone irritates the respiratory
system, aggravates asthma and bronchitis, and it inflames the lining
of the lungs. It harms vegetation and agricultural crops, and it
damages rubber and other materials.

Ozone is the major component of smog. It is produced from nitrogen
oxides and volatile carbon-based compounds when there is intense solar
radiation (energy), above all in the spring and summer. See The
Physics and Chemistry of of Ozone.
For more information: see the Environmental Protection Agency's page

Atmospheric concentration of ozone depends on the concentration of both
volatile organic carbon VOC and nitrous oxides NOx. Notice that
ozone concentration (the contours) decrease as VOC increases if NOX
concentrations are high, but that it increases if VOX concentrations
are low. Thus for some cities, decreasing NOx emissions leads to higher
ozone concentrations.
From National Academies Press (1991)Rethinking
the Ozone Problem in Urban and Regional Air Pollution.

By power plants that burn fuel containing sulfur (mainly coal and
oil),

By smelters, petroleum refineries, iron and steel mills, and by other
industrial processes. Sulfur dioxide emissions from copper smelters
in Peru as observed by sensors on NASA's Aura satellite. Click on image
for a zoom.
From Goddard Space Flight Center Science page on Top
Ten Smelters.

Additionally, plankton emit dimethylsulfide (DMS), which is oxidized
to form SO2.

Fuel combustion, largely from coal-fired power plants, accounts for
most of the total SO2 emissions in
the USA. But, thanks to the Clean Air Act, the national average of
ambient SO2 concentrations have decreased 52 percent from
1982 to 2001 accorning to the EPA.
Concentrations are also decreasing in Europe, but they are increasing
in other parts of the world, especially Asia.

Sulfer dioxide readilly dissolves in water or water vapor to form
acid, which makes the rain acidic. It is oxidized by OH· in
gas phase, or in cloud droplets to form sulfate aerosol (particles),
which makes the air hazy and causes dry deposition of acid.

Particulates are small particles (aerosols) suspended in the air.
The can be composed of:

Sulfates from SO2,

Nitrates from NO2,

Dust (dust storms),

Soot from diesel engines and fires (smoke), including burning of
agricultural waste, and forest fires)

Carbon-based molecules from industry or plants (think of the Great
Smoky Mountains),

Sea salt (from breaking ocean waves).

Particulate matter is divided into fine particles less than
2.5 micrometers in diameter PM2.5, and coarse particles between 2.5 and
10 micrometers in diameter PM10. The smaller particles are most harmful
because they can infiltrate deep into lungs. Smoke irritates eyes and
lungs.

They reflect sunlight. This increase the amount of sunlight reflected
back to space, cooling earth's surface.

They cause clouds to last longer. Because clouds reflect sunlight,
this too increases the amount of sunlight reflected
back to space, further cooling earth's surface.

They influence precipitation.

They reduce rainfall because small cloud droplets fall slower than
larger droplets.

Rain in hilly regins can be reduced by 30–50% during hazy conditions,
with visibility less than 8 km at hilltops (Rosenfeld et al. 2007).

There is mounting evidence that land surface
properties and atmospheric aerosol particles have a profound impact
on earth's thermodynamic and radiative energy budgets by affecting
precipitation processes. Large concentrations of sub-micron aerosols
act as cloud condensation nuclei (CCN) that produce large concentrations
of small cloud drops which are slow to coalesce into raindrops. The
decrease in cloud drop size also delays the formation of ice in the
cloud to lower temperatures. This can lead to suppression of precipitation
in shallow and short-lived clouds, as during winter over topographic
barriers, and subsequently to decreased water resources in semi-arid
regions. Manifestations of these processes are observed in all clouds,
from the shallow marine stratocumulus, through the modest rain clouds
over land in the extra-tropics, to the deepest tropical convective
clouds over the Amazon and equatorial oceans. From iLEAPS
Newsletter, #2, May 2006.
This image, made from data collected by the MODIS instrument on the Terra
satellite shows the relationship between aerosols and cloud formation.
It was collected off the coast of california on 26 June 2003 at 19:40
GMT. The colors correspond to size of cloud drops. The long blue tracks
are clouds composed of very small drops condensing on aerosols from ship
exhaust. Notice that when ship tracks cross naturally occuring clouds
(orange to black areas), they change the size of the cloud drops in these
clouds. From From iLEAPS
Newsletter, #2, May 2006.

Carbon monoxide

Carbon monoxide is a colorless, oderless, poisonous gas produced by
the incomplete burning of cabon in fuels, mainly by cars
and trucks and also by forest fires and burning of agricultural waste.
In the USA, about 77% of the pollutant comes from transportation sources.
In cities, as much as 95 percent of all CO emissions may come from automobile
exhaust. according to the EPA.
It is a deadly poison which binds to hemoglobin molecules in blood,
reducing the amount of oxygen carried to body tissues. Emissions and
concentrations of carbon monoxide have dropped 65% and 41% respectively
from 1982 to 2002 in the USA

Map of average concentration of carbon monoxide in the spring. From
Measurements Of Pollution In The Troposphere MOPITT instrument on the
Terra satellite. Note the high concentrations in the northern hemisphere
associated with industrial activity, and high concentrations in Africa
associated with agricultural burning. Click on the image to go to a
NASA website showing a 5.6 MByte animation
of the movement of carbon monoxide around the world in 2000 from
the Total Emissions Spectrometer on the Aura satellite.

In the past, automotive sources were the major
contributor of lead emissions to the atmosphere. As a result of EPA’s
regulatory efforts to reduce the content of lead in gasoline, however,
the contribution of air emissions of lead from the transportation sector,
and particularly the automotive sector, has greatly declined over the
past two decades. Today, industrial processes, primarily metals processing,
are the major source of lead emissions to the atmosphere. The highest
air concentrations of lead are usually found in the vicinity of smelters
and battery manufacturers. From EPA

It appears that lead is becoming less and less of an air
pollutant. "The 2002 average air quality concentration for lead
is 94 percent lower than in 1983. Emissions of lead decreased 93 percent
over the 21-year period 1982–2002." Only two areas, in Montana
and Missouri were in noncompliance with EPA standards in March 2006.

Rosenfeld, D., J. Dai, et al. (2007). Inverse Relations Between
Amounts of Air Pollution and Orographic Precipitation. Science 315
(5817): 1396-1398.
Particulate air pollution has been suggested as the cause of the recently
observed decreasing trends of 10 to 25% in the ratio between hilly and
upwind lowland precipitation, downwind of urban and industrial areas.
We quantified the dependence of this ratio of the orographic-precipitation
enhancement factor on the amounts of aerosols composed mostly of pollution
in the free troposphere, based on measurements at Mt. Hua near Xi'an,
in central China. The hilly precipitation can be decreased by 30 to 50%
during hazy conditions, with visibility of less than 8 kilometers at
the mountaintop. This trend shows the role of air pollution in the loss
of significant water resources in hilly areas, which is a major problem
in China and many other areas of the world.